Add a "Public" group

[adehnert]

It might be nice to have a "public" group on either the prod DB or one of the development instances that people could use to see what the group officer UI looks like. (Useful to, among other things, for a former officer to give instructions for using the DB to a new officer, if they don't remember exactly how things work.)

One approach would be to wait until #84 is fixed (and maybe a per-group officer cap override feature is added), and sync the Group Admin role to a public moira list. Another approach would be to have a little web form (in the DB or not) that allows adding yourself.